This invention relates to a laser oscillator that uses as a laser medium a single-crystal material doped with titanium, typically titanium-doped sapphire which is generally referred to as "Ti:sapphire". More particularly, this invention relates to an improved laser oscillator that uses Ti:sapphire as a laser medium and that is adapted to perform turnable operation so as to produce a single-pulsed laser output at a large peak power in a consistent way.
Lasers capable of producing a pulsed output of short duration at a high peak power are suitable for use in precision measurements, evaluation of high-speed operations, etc. In particular, lasers that use Ti:sapphire as a laser medium are tunable in wavelength and suitable for use in spectroscopic measurements.
Ti:sapphire has long been used as a medium for tunable lasers. Dye lasers that use dyes dissolved in liquid solvents as laser media are also tunable in wavelength. However, the range of wavelengths that can be tuned with dye lasers is no more than about 50 nm, which is smaller than the range (500 nm) that can be tuned with Ti:sapphire. In addition, it is difficult to generate large-energy output oscillation with dye lasers. Further, dye lasers which often use toxic fluids are not easy to handle and their running cost is undesirably high.
Conventional pulsed lasers of short duration that use Ti:sapphire as a laser medium adopt a continuous-wave (CW) operating argon ion laser as a pumping (light) source and they have principally been operated with the internal cavity modes being locked by means of an electrooptical (EO) device, an acoustooptical (AO) device or some other suitable device that is placed within the cavity. Mode locking is a technique for allowing a laser to be oscillated in the sole internal lasing mode. Laser light travels back and forth many times through the cavity and by insuring that an optical device in the cavity will cause a smaller optical loss when a single laser pulse of short time duration (i.e., lasing mode) passes through it than in other cases, laser light of short pulses can be selectively generated. Examples of optical devices used for mode locking purposes include EO and AO devices.
Obviously, in order to obtain laser light of short pulses by mode locking, a high-frequency signal must be applied to the optical device of interest at time intervals that correctly match the passage of an oscillating laser pulse through the device. The frequency of the signal is the inverse of the time required for the oscillating light to travel through the cavity in one forward or return path. Stable pulsed laser light cannot be obtained unless the correct value of frequency is selected. Even if it is possible, the waveform of the obtained pulse will often contain sags (or droops) and other distortions that would be absent if an ideal temporal waveform were obtained. To avoid this problem, precision devices are required but then the construction of the equipment becomes complex.
Generally, a light source capable of emitting continuous radiation is desirably used as an excitation light source if mode locking is to be performed. If a source of non-continuous pumping light is to be used, either of the following two methods may be adopted. One method is using pulsed light that is applied repetitively at the same frequency as the one discussed above. In this case, the input pumping light has the same frequency as the signal to be applied to the optical device of interest but the resulting time lag makes it even more difficult to apply the signal on well-timed relationship. As a further problem, it is also considerably difficult to obtain a light source capable of emitting light at the desired high frequency. According to the other method, the time duration of each pulse from the pumping light source is extended to provide quasi continuous radiation so that more than one laser light of short pulses can be obtained within the time period of application of the input pulse. If this method is adopted, there is no need to match the timing of input exciting light with that of the application of a signal to the optical device of interest. However, individual laser pulses have different intensities of light.
Lasers that constitute Ti:sapphire as a laser medium may adopt adw argon ion laser, a CW Nd:YAG laser and some other CW light sources. However, with these lasers, the maximum efficiency of energy conversion from the input electric power to the pumping laser light that can be achieved is only about 1%, and energy conversion of the pumping light to oscillating lights in the Ti:sapphire laser oscillator is no more than 1%. Therefore, the overall efficiency of energy conversion from input electric power to the Ti:sapphire laser light is less than 1.times.10.sup.-4. Another drawback of this method is that it is incapable of producing laser light of high intensity. Further, the need to combine two units of cavity increases the complexity of the optical system in the equipment, which unavoidably causes an increase in its size.
A Ti:sapphire laser oscillator that uses a flash lamp or some other light source of non-continuous radiation as an exciting light source is capable of producing pulsed light in a convenient way by conventional oscillation methods. However, "spiking" (the generation of pulses each containing more than one peak) occurs to producing only laser light that has a comparatively small peak energy.
In "side-pumping" where a laser medium is excited from its lateral side by means of a pumping light source such as a flash lamp, the greater part of the energy of the exciting light fails to be used in excitation of the laser medium, whereby only laser light of lower efficiency of optical energy conversion is obtained. A side-pumping sapphire laser oscillator is capable of producing pulsed light in a convenient way by conventional oscillation methods.
A simplified version of laser oscillator has been proposed that uses a laser medium such as a Nd-doped single-crystal material (e.g. ND:YAG) and that is excited with a high-output semiconductor laser. The major advantage of this laser oscillator is its ability to produce a peak energy during pulse oscillation. However, the wavelength (typically in the range of 1000-1100 nm) at which this laser oscillator can operate is not tunable although it is attainable with semiconductor lasers. As a further problem, semiconductor lasers which operate at wavelengths of 700 nm and longer are not suitable for use as light sources to excite Ti:sapphire whose excitation wavelength lies in the neighborhood of 500 nm.